The invention relates to a friction pump for conveying gases which is designed in accordance with Siegbahn principles (further Siegbahn pump), and has at least one stage formed by a disc rotatable between two stationary discs, with the sides of the stationary discs facing the rotatable disc being provided with spirally shaped grooves and with webs.
A set of magnetic bearings forms an integral component of the pump. Herein the components characteristic of the magnetic bearings are the same as those which are significant for the functioning of a vacuum pump.
Friction pumps for conveying gases of the most varied types of construction are known. Their mode of operation is based on the transmission of impulses of moving walls to the gas particles. In this way, a gas flow in the desired direction is produced. Friction pumps which operate in a pressure range where the free travel lengths of the gas molecules is large compared to the geometrical dimensions of the pump, i.e., in the molecular flow region, are called molecular pumps.
The first friction pump of this type was described in an article by W. Gaede in "Ann. Physics 41(1913), 337p. A modification of the pump of Gaede, without deviating from the basic principle, is disclosed in an article by Siegbahn, Mathematical Archives, Ash. FIGS. 30B (1943).
In Siegbahn pumps, a rotating disc is used here as the moving wall. Another variant of a friction pump is disclosed in an article by F. Holweck, Compte Rendue Academy of Science 177 (1923) 43p. Here a cylinder surface is used as a moving wall.
A further modification of gas friction pumps is disclosed in an article of W. Becker, Vacuum Technology, 9/10 (1966). In this case, moving and stationary walls are alternately disposed one after another, wherein both wall consist of turbine-shaped discs provided with vanes. For this reason, the name turbomolecular pump was introduced for this type of pump.
These modifications of the friction pump first introduced by Gaede are evermore important in vacuum technology, in particular, in high--and ultra high vacuum technology. The utilization regions of the turbomolecular pump based on Becker principles, on the one hand, and the friction pump based on Siegbahn principles and that on Holweck principles on the other hand, are different. The turbomolecular pump is able to provide a high pressure ratio due to its construction consisting of a plurality of stages arranged in a row and is thus particularly suitable for use in the high and ultra high vacuum area. Its utilization is however, limited in the direction of higher pressures, since it is only fully efficient at low pressures approximately less than 10.sup.-1 mbar because of the large spacings of the pump parts. The friction pumps according to Siegbahn and those according to Holweck are eminently suitable for being used in the adjacent pressure range towards the high side. They can be either used separately in this pressure range, as well as also arranged in series with a turbomolecular pump. This last combination of a turbomolecular pump and a molecular pump represents an elegant possibility of moving the working range of a turbomolecular pump in the direction of higher output pressures.
It is essential for the functioning of this type of pump, that the spacing between rotating and stationary components be very small in order to keep flowback and retro-circulation losses as small as possible. This applies especially to friction pumps according to Siegbahn and Holweck. To this it must be added, that these pumps (as well as the turbomolecular pumps) only operate in the higher pressure region in the molecular flow range, if the spacings between the rotating and stationary components fulfill the condition that they be small compared to the average free travel length of the molecules of the gas to be pumped, because these pumps develop their entire pressure ratio only in the molecular flow region.
A common feature of these pumps consists in that the pressure ratio depends exponentially and the suction capacity linearly upon the circumferential velocity of the rotating parts which serve as moving walls. Therefore, it is essential for an efficient pumping behavior to raise the rotor rpm to an extent still compatible with a dependable and continuous operation.
Herein the rotor bearing system is particularly important. Apart from the classical mechanical rotor bearings, i.e. lubricated ball bearings, a combination of permanent magnet bearings and ball bearings is used today. Actively controlled magnetic bearings of the most varied embodiments are used for completely contactless bearing support.
The two extreme requirements, high rpm and minimum spacings between stationary and rotating parts, are two conditions for the design of a friction pump which can be combined with each other only with great difficulty. The higher the rpm the larger must be the minimum spacing between stationary and rotating parts in order to prevent butting wear and thus damage or even complete destruction of the pump. In the friction pumps of Siegbahn and Holweck, where the gaps for achieving an effective and an efficient pump behavior must be extremely narrow, these criteria are of utmost significance. In particular, the preservation of the predetermined gaps over a long period of time during continuous operation is fraught with great difficulties.
The object of the invention is a friction pump which assures a secure and dependable continuous operation at the extreme requirements, such as high rpm and minimum gaps, while preserving the outstanding properties in vacuum technology, such as for instance a vacuum devoid of oil, which is characteristic of such pumps. Another object of the invention is a friction pump in which extremely tight gaps are maintained over the time span of a continuous operation of friction pumps of Siegbahn and Holweck.